The present invention is directed to a method and composition for use in inhibiting the formation and deposition of coke on surfaces and corrosion of these surfaces during the elevated temperature processing of hydrocarbons. Coke deposition and corrosion are generally experienced when hydrocarbon liquids and vapors contact the hot metal surfaces of the processing equipment. While coke formation is perhaps not entirely technically understood, because of the complex makeup of the hydrocarbons, the hydrocarbons at elevated temperatures and in contact with hot metallic surfaces undergo various changes through either chemical reactions and/or decomposition of various unstable components of the hydrocarbon. The undesired products in many instances include coke, polymerized products, deposited impurities and the like. Whatever the undesired product that may be formed, the result is the same, i.e., reduced economies of the process. If these deposits are allowed to remain unchecked, heat transfer, throughput and overall productivity are detrimentally affected. Moreover, downtime is likely to be encountered due to the necessity of either replacing and/or cleaning of the affected parts of the processing system.
While the formation and type of undesired products are dependent upon the hydrocarbon being processed and the conditions of the processing, it may generally be stated that such products can be produced at temperatures as low as 100.degree. F.; but are much more prone to formation as the temperature of the processing system and the metal surfaces thereof in contact with the hydrocarbon increase. At these temperatures, coke formation is likely to be produced regardless of the type hydrocarbon being charged. The type coke formed, i.e., amorphous, filamentous or pyrolytic, may vary somewhat; however, the probability of the formation of such is quite high.
As indicated in U.S. Pat. Nos. 3,531,394 and 4,105,540 the teachings of which are incorporated herein by reference, coke formation and deposition are common problems for example in ethylene (olefin) plants which operate at temperatures where the metal surfaces in contact with the hydrocarbon are at 1600.degree. F. and above. The problem is prevalent in the cracking furnace coils as well as in the transfer line exchangers where pyrolytic type coke formation and deposition is commonly encountered. Ethylene plants, often referred to generally as "olefin plants", originally produced simple olefins such as ethylene, propylene, butenes and butadiene from a feed of ethane, propane, butanes and mixtures thereof. Later developments in the area of technology, however, have led to the cracking of heavier feedstocks because of their availability to produce aromatics and pyrolysis gasoline as well as light olefins. Feedstocks now include light naphtha and gas oil.
According to the thermal cracking processes utilized in olefin plants, the feedstocks in olefin plants are cracked generally in the presence of steam in tubular pyrolysis furnaces. The feedstock is preheated, diluted with steam and the mixture heated in the pyrolysis furnace to about 1500.degree. F. and above, most often in the range of 1500.degree. to 1650.degree. F. The effluent from the furnace is rapidly quenched by direct means or in exchangers which are used to generate high pressure steam at 400 to 800 psig for process use. This rapid quench reduces the loss of olefins by minimizing secondary reactions. The cooled gas then passes to the prefractionator where it is cooled by circulating oil streams to remove the fuel oil fraction. In some designs, the gas leaving the quench exchanger is further cooled with oil before entering the prefractionator. In either case, the heat picked up by the circulating oil streams is used to generate steam and to heat other process streams. The moisture of gas and steam leaving the prefractionator is further cooled in order to condense the steam and most of the gasoline product in order to provide flux for the prefractionator. Either a direct water quench or heat exchanger are used for this cooling duty.
After cooling, cracked gas at or close to atmospheric pressure is compressed in a multistage compression system to much higher pressures. There are usually four or five stages of compression with interstage cooling and condensate separation between stages. Most plants have hydrocarbon condensate stripping facilities. Condensate from the interstage knockout drums is fed to a stripper where the C.sub.2 hydrocarbons and lighter are separated. The heavier hydrocarbons are fed to the depropanizer.
While various treatments have been proposed to eliminate or reduce pyrolytic filamentous coke formation at elevated temperatures, none have attained any great degree of success. In the book "Coke Formation on Metal Surface" by Albright and Baker, 1982, methods are described which utilize silicon and aluminum as pretreatments. In accordance with the procedure, the furnace tubes are pretreated with silicon and aluminum hours before introduction of the hydrocarbon feedstocks. With the use of silicon, furnace tubes are coated by the chemical vaporization of an alkoxysilane. While U.S. Pat. Nos. 4,105,540 and 4,116,812 are generally directed to fouling problems in general, the patents disclose the use of certain phosphate and phosphate and sulfur containing additives for use purportedly to reduce coke formation in addition to general foulants at high temperature processing conditions.
With respect to coke retardation, various efforts have been reported, namely:
1. French Pat. No. 2,202,930 (Chem. Abstract Vol. 83, 30687K) is directed to tubular furnace cracking of hydrocarbons where molten oxides or salts of group III, IV or VIII metals (e.g., molten lead containing a mixture of K.sub.3 VO.sub.4, SiO.sub.2 and NiO) are added to pretreated charge of, for example, naphtha/steam at 932.degree. F. This treatment is stated as having reduced deposit and coke formation in the cracking section of the furnace.
2. Starshov et al, Izv Vyssh. Uchebn. Zaved., Neft GAZ, 1977 (Chem. Abst. Vol. 87: 154474r) describes the pyrolysis of hydrocarbons in the presence of aqueous solutions of boric acid. Carbon deposits were minimized by this process.
3. Nikonov et al., U.S.S.R. No. 834,107, 1981; (Chem. Abst. 95: 135651v) describes the pyrolytic production of olefins with peroxides present in a reactor, the internal surfaces of which have been pretreated with an aqueous alcoholic solution of boric acid. Coke formation is not mentioned in this patent since the function of the boric acid is to coat the inner surface of the reactor and thus decrease the scavenging of peroxide radicals by the reactor surface.
4. Starshov et al., Neftekhimiya 1979 (Chem. Abst: 92: 8645j) describes the effect of certain elements including boron on coke formation during the pyrolysis of hydrocarbons to produce olefins.
5. U.S. Pat. No. 2,063,596 discusses in its prior art section the problems associated with the processing of hydrocarbons in equipment whose metallic parts have been supplied with a metalloid. The general impression is that such has not been utilized successfully.
6. U.S. Pat. No. 1,847,095 describes the metalloid compounds that are capable of yielding "volatile hydrogen" during the processing of hydrocarbons. The teachings of the patent are further limited in that reaction vessels composed of "Substances (free iron in particular) leading to deposition of soot should be absolutely excluded from the hot parts of the apparatus". In addition, although inner walls may be made of materials containing metalloids (quartz, ferro-silicon, mica and porcelain), addition of the hydrides of a metalloid are still required to inhibit coke formation. In fact, in the presence of silica gel as an example in that patent, hydrides of silicon were added to reduce coke formation. The patent is also notably silent with regards to the type of coke encountered and the problems associated therewith.
7. Baker, R. T. K., Gas Chem. Nucl. React. Large Indust. Plant, Proc. Conf., 1980. Chem. Ab. Vol. 94, 1981, 94: 814h, is directed to the role of various additives, e.g., B.sub.2 O.sub.3 in effecting the growth rate of filamentous coke produced from the decomposition of C.sub.2 H.sub.2 on Ni-Fe or Mo Catalysts. B.sub.2 O.sub.3 is stated as being the only additive which failed to provide any significant reduction in the growth of the filaments.
In spite of the above, the industry's requirements for a cost-effective method to inhibit coke formation which was safe to handle, could be easily and conveniently added, and which would provide additional benefits as a corrosion inhibitor were never satisfied.